Aging is inevitable. It is governed by both inheritance and environmental factors. A deeper understanding of the aging process might allow us to slow its progression or at least delay the onset of age-associated diseases, and thus extend the well being of individuals. The universality in the decline of energy with age highlights energy metabolism and the role of role mitochondria (mt) in aging. A widely accepted but still unproven theory of aging centers on the accumulation of cellular damage by the generation of reactive oxygen species. In cells, reactive oxygen species are generated in mitochondria and have been demonstrated to damage mitochondrial DNA. Since DNA repair is limited in mitochondria, some of this damage may go on to cause mutations when the DNA is replicated. We have developed, established, and validated exceptionally sensitive assays to quantify mutations in nuclear and mitochondrial DNA. Our specific aims will be focused in two directions: 1) We will determine the frequency and types of mutations that increase in different tissues during aging in humans. We will analyze the mechanism by which specific mitochondrial mutations are be selective amplified. 2) Our focus will be on Parkinson syndrome, one of the most prevalent age-dependent neurological diseases. Using cell culture and mouse models, we will examine the contribution of reactive oxygen species to mitochondrial mutagenesis. Most importantly, we will ascertain if selectively amplified mitochondrial mutations can provide a marker for diagnosis of Parkinson syndrome and or monitoring of disease progression and response to treatment.